The present invention relates to a projection lens system for projecting images, and more particularly to a projection lens system for a video projector which combines images displayed on a plurality of monochrome cathode ray tubes, and then enlarges and projects the combined images on a large screen.
A general cathode ray tube (CRT) for televisions is the most well-known image display apparatus. If the television CRT is a direct-viewing type, a thirty-inch screen is generally the practical limit for the screen size. Beyond these dimensions, the screen's luminance and contrast are lowered, and resolution is degraded. Moreover, there are other technical limitations which limit the size of a CRT.
However, for an image projection apparatus which employs a projection lens, such technical limitations of the CRT can be overcome by performing such operations as aberration correction of the projection lens. Accordingly, larger images of a better picture quality can be displayed. Such an image projection apparatus has already been employed in a projection TV, video projector, etc. In the future, image projection apparatuses will be extended to computer peripheral apparatuses, video conference systems, and other similar applications.
Generally, an image projection apparatus includes a combination of projection type CRTs and lens components for enlarging and projecting images generated from projection type CRTs. Also, to project color images, three monochrome CRTs of red, green and blue are used. In this case, there are two different types, according to the disposition of the CRTs.
One of these two types is a triple-CRT triple-lens system which is a combination of three monochrome CRTs and three lens units whose respective optical axes are arranged to face the center of a screen. However, since the location for each of the CRTs and lens units, needs to be adjusted adjusting the focus and magnitude of the image displayed on the screen is difficult.
The other of the two types is a triple-CRT single-lens system which is a combination of three monochrome CRTs (red, green and blue) arranged in a T-shaped array and having two dichroic mirrors which are arranged in an X-shaped array at the point where the respective optical axes intersect, and a single lens unit which is positioned in front of the mirrors for enlarging and projecting a color image to the screen. In this type of system, the screen control is easier and the structure is simpler than those of the above system. However, if a 5-inch CRT is used in this type of system, since the CRTs are spaced apart from the single lens unit by about 130-150 mm, aberration occurs and an adequate quantity of peripheral light is difficult to secure. In order to overcome such difficulties, a large-diameter lens or a CRT with a concave image-generating surface has been used. However, the large lens increases the dimension of the apparatus, and the concave image-generating surface makes CRT manufacturing quite complicated.
Accordingly, in the triple-CRT single-lens system, it is desirable to attain both a large aperture with a small lens and flatness of the CRT's image-generating surface.
On the other hand, the CRT used in the image projection apparatus is usually smaller than the general type. Thus, a temperature rise results from the high-power electron beams being concentrated on the small image-generating surface. This may not only change the light emitting characteristic of the fluorescent material on the image-generating surface, but also may damage the glass surface. Therefore, the cooling structure must be considered in view of the thermal characteristics of the CRT.
As an example of a conventional triple-CRT single-lens system, U.S. Pat. No. 4,764,806 describes a structure wherein a plano-convex lens having the same radius of curvature as that of the concave image-generating surface of a CRT is used. Here, the lens and CRT peripheries are tightly sealed, so that the space thereby formed can be filled with an oil having the same refractive index as that of the face glass of the CRT therein, so as to remove optical aberrations such as spherical aberration, coma aberration, etc. Also, according to U.S. Pat. No. 4,842,394 optical aberration can be eliminated by means of two spherical lenses and an aspherical lens.
However, difficulty still exists in the manufacture of the CRTs of the above U.S. Patents since the lenses employed are designed on the basis of the concave image-generating surface of the CRT. In addition, even though the above-mentioned oil serves as a CRT coolant, such a structure is unfavorable to CRT control for display adjustments.